Speech codec is typically based on Coded Excited Linear Prediction (CELP). FIGS. 1 and 2 schematically illustrate typical implementations of an adaptive codebook and a fixed codebook, respectively, used for constructing an excitation signal of speech. Although the CELP technique can approximate practical speech, some distortions of synthesized speech signal inevitably exist. Especially in low bit-rate speech coding, the distortion can be quite severe, and thus requiring post-processing of decoded speech signal.
Traditional post-processing techniques in AMR-WB and AMR-WB+ codec include pitch emphasis, frequency-selective pitch enhancement, etc., some of which are designed to reduce pitch distortion due to inadequate bits under low bit-rate conditions. Current post-processing techniques for pitch enhancement can be divided into two categories. One technique is to divide the input signal into multiple frequency bands and then to enhance pitch components of speech in certain frequency bands but not all frequency bands. The output of post-processing signals is the summation of signals from all the bands. One disadvantage of this technique is that the application of multiple bandpass filters requires a large computation burden. The other technique is to directly add the adaptive codebook driven excitation into total excitation. Applying this technique requires computing certain internal parameters using multiplications and square computations, and thus causing excessive computational complexity.